1. Field of the Invention
The present invention relates to a driving mechanism for driving a lens unit constituting an image pickup optical system in e.g. a camera phone for zoom control, focus control, or a like control, as well as a driving device incorporated with the driving mechanism, and a lens driving device incorporated with the driving mechanism.
2. Description of the Related Art
In recent years, a high-resolution image pickup device has been developed. For instance, the number of pixels of an image sensor to be loaded in a camera phone or a like device has been spectacularly increased. As the high-resolution image pickup device has been developed, a focus function, a zoom function, or a like function has been demanded, in addition to a basic function, i.e., an image shooting function.
A lens driving device for moving a lens in the optical axis direction is necessary to impart the aforementioned additional functions to the camera phone. In recent years, various applications of a lens driving device incorporated with a shape memory alloy (hereinafter, called as “SMA”) actuator have been proposed. The lens driving device is constructed in such a manner that a contraction force is generated by energizing and heating the SMA actuator, and the contraction force is utilized as a lens driving force. Generally, a lens driving device incorporated with an SMA actuator is advantageous in easily reducing the size and the weight of the lens driving device, or obtaining a relatively large magnitude of force.
As a lens driving mechanism incorporated with an SMA actuator, for instance, there are known structures as disclosed in e.g. Japanese Unexamined Patent Publication No. 2005-195998 (D1), Japanese Unexamined Patent Publication No. 2002-130114 (D2), and Japanese Unexamined Patent Publication No. 2005-156892 (D3). D1 discloses an arrangement, wherein an SMA wire is spirally wound around an outer surface of a cam cylinder for guiding a lens in the optical axis direction to pivotally move the cam cylinder by a contraction force of the SMA wire. D2 discloses an arrangement, wherein an SMA wire is wound between a lens frame and a fixed portion to move the lens frame by a contraction force of the SMA wire. D3 discloses a lens driving mechanism including a mechanism for magnifying a contraction force of an SMA wire with use of a gear mechanism.
In the case where a high-performance focus function, a high-performance zoom function, or a like high-performance function is provided in a camera phone or a like device, it is necessary to greatly and stably displace a lens while reducing the size and the weight of the lens driving mechanism. Generally, however, a contraction displacement amount obtainable by energizing and heating an SMA wire is at most about several percentages with respect to the entire length of the SMA wire. Considering durability of the SMA wire, an allowable displacement range is at most about 3%. Accordingly, in the approach of directly driving a lens, i.e., a lens frame by an SMA actuator, as proposed in the arrangements of D1 and D2, a movable distance of a lens is relatively small, and it is difficult to obtain a lens moving amount required in high-performance auto-focusing or high-performance optical zooming, despite the advantages that stable lens displacement is secured, and the size and the weight of the lens driving mechanism are reduced. In the arrangement of D3, although a certain lens moving amount is secured, it is difficult to reduce the size and the weight of the lens driving device, because loading a gear mechanism is necessary.